Astronomers Confirm the Existence of Failed Stars

By JOHN NOBLE WILFORD

Published: November 30, 1995

There are stars and there are planets, and now astronomers are more certain than ever that there is something in between, elusive dim objects known as brown dwarfs that never made it to stardom.

Astronomers working at Palomar Observatory in California have produced the first photograph of one of these mysterious objects, sometimes called failed stars, and also detected the infrared signature of methane there, evidence that the object is much too cool to be a real star. Their findings were confirmed this month with other pictures and measurements made by the Hubble Space Telescope.

After three decades of theorizing and several tantalizing but not entirely convincing observations, astronomers think they have finally found unambiguous evidence for one of these brown dwarfs, something with too little mass to generate the nuclear fusion to shine like the Sun and other stars and yet too massive and hot to be a planet, even one as huge as Jupiter.

The newly discovered brown dwarf looks remarkably like Jupiter, but is 20 to 50 times more massive. It is one-250,000th as bright as the Sun, much cooler than any star, and may be the faintest object ever seen in the company of another star. The object is a companion to the small star Gliese 229, which is 19 light-years away in the constellation Lepus, and so has been designated GL229B.

The discovery is being reported today in the British journal Nature and tomorrow in the journal Science. The confirming work by the Hubble telescope and other details were described yesterday in a news conference at the National Aeronautics and Space Administration in Washington.

"It is undeniably a brown dwarf," said Dr. Shrinivas Kulkarni, an astronomer at the California Institute of Technology in Pasadena and one of the discoverers.

Other scientists reacted to the reports with unusually strong affirmations that it appeared that a brown dwarf, which gets its name for its dimness and relatively small size, had indeed been identified. In the past, reports of brown-dwarf candidates often drew guarded comments and outright skepticism. And most of them did turn out to be something else, like a dim star, not a brown dwarf. A few could still be confirmed as brown dwarfs.

In an article about the discovery in Science, Dr. Adam Burrows, an astrophysicist at the University of Arizona in Tucson, said flatly, "The thing is a brown dwarf."

Dr. Stephen P. Maran, an astronomer at the Goddard Space Flight Center in Greenbelt, Md., said in an interview: "There's no question it's a brown dwarf. This object removes any remaining doubts about the existence of brown dwarfs."

The significance of the discovery extends beyond the recognition of a new class of cosmic phenomena, though that in itself is exciting for astronomers to contemplate.

"We are finding a wider variety of phenomena than we ever dreamed of," said Dr. Anne L. Kinney, an astronomer at the Space Telescope Science Institute in Baltimore. "The universe is much more diverse than the solar system we live in."

Scientists also expect that observations of brown dwarfs could help them determine the boundaries of mass, temperature and other physical and chemical properties for true stars and perhaps the different processes that lead to the formation of planets or brown dwarfs, instead of stars. The techniques for searching out brown dwarfs and examining their characteristics could also be applied to the search for planetary systems around other stars. With GL229B as a model, scientists should be better able to distinguish between massive Jupiter-like planets and brown dwarfs orbiting other stars.

Another reason scientists had been searching for brown dwarfs was to see if these faint objects could be plentiful enough to account for a significant portion of the "missing mass" in the universe -- that is, the more than 90 percent of the supposed mass that remains undetectable. The missing mass is sometimes called dark matter. But in their three-year search, astronomers so far could find only one brown dwarf out of 100 nearby stars examined.

"I don't think we are going to find a lot of brown dwarfs out there, and I don't think they are going to solve the dark matter question," Dr. Kinney said.

The first sightings of the brown dwarf were made a year ago by astronomers from Caltech and Johns Hopkins University in Baltimore. Using advanced sensors and optics attached to a 60-inch telescope at Mount Palomar, they detected a dim object about four billion miles from the star GL229, or about the distance the planet Pluto is from the Sun. The object's low temperature and luminosity, no more than one-tenth that of the least luminous hydrogen-burning star, led the astronomers to conclude that they had for the first time found and photographed a brown dwarf.

The members of the discovery team, with Dr. Kulkarni, are Dr. Tadashi Nakajiim, Dr. Benjamin R. Oppenheimer and Dr. Keith Matthews, all of Caltech, and Dr. David A. Golimowski and Dr. Samuel T. Durrance of Johns Hopkins. Their work was summarized in Nature.

The first sighting was followed up on two nights of observations in September with Palomar's 200-inch Hale telescope, equipped with special optics and an infrared camera. In this way, Dr. Kulkarni and other astronomers, all of Caltech, did the first analysis of light from a cool brown dwarf, finding traces of methane in the object's atmosphere.

As Dr. Takashi Tsuji of the University of Tokyo had recently determined, molecules like methane are formed readily in the low temperatures of planets and old brown dwarfs, but could not exist in the intense heat of a nuclear-burning star. Finding methane in the brown dwarf, the astronomers realized, was a sure sign of low temperatures and thus of an object that fell short of being a real star.

Scientists estimate that a star forms if the coalescing gases compress to a mass equivalent of at least 80 Jupiters, or about 8 percent the mass of the Sun. In youth a brown dwarf may glow hot from the gravitational energy of the compressing interstellar gases, but if critical mass is not reached and fusion begun, in time it will cool off, leaving a dim glow from residual gravitational energy.

The detection of methane "was a great revelation," Dr. Kulkarni said, and then on Nov. 17, the orbiting Hubble telescope took pictures that seemed to remove any doubts.

The methane results were described in the Science article by Dr. Kulkarni and his Caltech colleagues. These astronomers were joined in interpreting the Hubble pictures by Dr. Christopher Burrows, an astronomer at the space telescope institute. Other observations at Palomar and by the space telescope are being planned.